1. Field of the Invention
The present invention relates to a planar light source device from which light is shined on the rear or back side of a display element such as a liquid crystal display, and to a display apparatus incorporating the same.
2. Description of the Related Art
As disclosed in, for example, Japanese Patent No. 3427636 (hereinafter called Patent Document 1), a conventional planar light source device is provided with a cone-shaped recess having a triangular cross section at the rear center of a light guide plate, in which recess a light source is disposed. In such a planar light source device as disclosed in Patent Document 1, the amount of light is small toward the front of the light source. However, light intensity at a portion of the above cone-shaped recess can be higher than that at the rest of portions, so that uniformities of intensity distribution cannot in some cases be achieved.
Furthermore, a planar light source as disclosed in Japanese Unexamined Patent Publication No. H10-82916 (hereinafter called Patent Document 2) is provided with a truncated cone-shaped recess for disposing the light source therein at the rear face of the light guide plate, and in the recess is disposed a light source such as an LED, and above the light source is placed a hyperbolic light reflective surface. However, a problem with such a planar light source device as disclosed in Patent Document 2 is that light emitted toward the front of the light source is mostly absorbed by the light reflective surface, thus resulting in reduction in light utilization efficiency. Still furthermore, in a liquid crystal display device as disclosed in Japanese Unexamined Patent Publication No. H10-104620 (hereinafter called Patent Document 3), a groove is formed on the bottom in the center of a light guide plate in parallel with longitudinal sides of the light guide plate, in the groove is accommodated a cold-cathode fluorescent lamp, and on the bottom of the groove is formed a light-transmission adjusting film. However, a problem with such a liquid crystal display device as disclosed in Patent Document 3 is that although direct light emitted from a cold-cathode fluorescent lamp is adjusted by the light-transmission adjusting film, the light is not diffused adequately, thus causing intensity irregularities at the front of the light source.
Still further, in a back light device as disclosed in Japanese Unexamined Patent Publication No. 2007-149451 (hereinafter called Patent Document 4), a light source hole part for accommodating the light source is provided on the rear side of a light guide plate, and a recess is provided on a light exit surface located opposite the hole part. The Patent Document 4 also discloses that the recess is of a cone shape, having a conical surface, combined with a cylinder; of a square pyramid shape, having a plurality of oblique faces, combined with a quadrangular prism; or of a hexagonal pyramid shape, having a plurality of oblique faces, combined with a hexagonal prism. However, a problem with such a backlight device as disclosed in Patent Document 4 is that because light emitted from the light source is transmitted radially, a part of the light, which does not propagates toward the oblique faces of the recess, exits in the proximity of the light exit surface, thereby causing intensity irregularities in the proximity of the light source.